usb: gadget: udc: core: fix NULL pointer dereference when handle disconnect

This patch fix NULL pointer dereference with the following log
when usb gadget is configured as uvc.

PC is at usb_gadget_disconnect+0x3c/0x4a
LR is at dwc3_gadget_pullup+0xllb/0xl28
 pc : [<b0lfa202>] lr : [<b0le67a7>] psr : 60000033
......
 [<b0lfa202>] (usb_gadget_disconnect) from [<b0lfa227>] (usb_gadget_deactivate+0xl7/0x34)
 [<b0lfa227>] (usb gadget-deactivate) from [<b0lf6e3b>] (usb function deactivate+0x3d/0x86)
 [<b0lf6e3b>] (usb_function_deactivate) from [<b0202421>](uvc_function_disconnect+0xd/0x30)
 [<b0202421>] (uvc_function_disconnect) from [<b0202a9b>](uvc_v412_release+0xlb/0x52)
 [<b0202a9b>] (uvc_v412_release) from [<b0210291>](v412_release+0x73/0x7e)
 [<b0210291>] (v412_release) from [<b007ea9f>] (_fput+0x43/0xc8)
 [<b007ea9f>] (_fput) from[<b00239db>](task_work_run+0x5d/0x76)
 [<b00239db>] (task_work_run) from [<b0016767>] (do_exit+0x2db/0x648)
 [<b0016767>] (do_exit) from [<b0017343>] (do_group_exit+0x33/0x84)
 [<b0017343>] (do_group_exit) from [<b00id2bf>](get_signal+0xl91/0x49a)
 [<b00ld2bf>] (get_signal) from [<b000delf>] (do_work pending+0x87/0x26e)
 [<b000delf>] (do_work pending) from [<b000828f>](slow_work _pending+0x9/0xl6)

Signed-off-by: William Wu <william.wu@rock-chips.com>
Change-Id: I838d62244e38a20f37ad9b530d5171c5e8de25c4

drivers/usb/gadget/udc/core.c

@@ -732,7 +732,8 @@ int usb_gadget_disconnect(struct usb_gadget *gadget)
 	ret = gadget->ops->pullup(gadget, 0);
 	if (!ret) {
 		gadget->connected = 0;
-		gadget->udc->driver->disconnect(gadget);
+		if (gadget->udc->driver)
+			gadget->udc->driver->disconnect(gadget);
 	}
 
 out: